linux-kernel - Re: [PATCH v4 06/15] ftrace: Use an opaque type for functions not callable from C

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Date:   Wed, 6 Oct 2021 17:10:16 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>, X86 ML <x86@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>, llvm@...ts.linux.dev
Subject: Re: [PATCH v4 06/15] ftrace: Use an opaque type for functions not
 callable from C

On Wed, 6 Oct 2021 13:43:35 -0700
Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> On Wed, Oct 06, 2021 at 10:45:41AM -0700, Sami Tolvanen wrote:
> > On Wed, Oct 6, 2021 at 9:58 AM Steven Rostedt <rostedt@...dmis.org> wrote:  
> > >
> > > On Wed, 6 Oct 2021 09:31:04 -0700
> > > Sami Tolvanen <samitolvanen@...gle.com> wrote:
> > >  
> > > > > > On Thu, Sep 30, 2021 at 11:05:22AM -0700, Sami Tolvanen wrote:  
> > > > > > > With CONFIG_CFI_CLANG, the compiler changes function references to point
> > > > > > > to the CFI jump table. As ftrace_call, ftrace_regs_call, and mcount_call
> > > > > > > are not called from C, use DECLARE_ASM_FUNC_SYMBOL to declare them.  
> > > > >
> > > > > "not called from C" is a bit confusing.  
> > > >
> > > > Any thoughts on how to make this less confusing?  
> > >
> > >  "Not called by C code, but injected by the compiler."
> > >
> > > ?  
> > 
> > Sure, sounds good to me. I'll update this in v5.  
> 
> "injected by the compiler" sounds even more confusing.  It almost sounds
> like those functions are generated by GCC, which they are most
> definitely not.
> 

Heh, I was thinking of the locations that are injected (mcount / fentry) as
these are just replacements for them. Those injections are added by GCC.

So, continuing the bikeshedding, what about "not called by C code, but are
trampolines injected as calls replacing the nops at the start of
functions added by the compiler." ?

-- Steve